June 21st marks the beginning of summer in the Northern Hemisphere and the
beginning of winter in the Southern Hemisphere. In the North it's the longest
day of the year. At mid-latitudes there is sunlight for over 16 hours. Above
the Arctic Circle the sun doesn't set at all!

Above:
The Earth's axis (the black line) is tilted by 23.5 degrees relative to the
plane in which our planet orbits around the Sun. That's why we have seasons.
For three months of the year, centered on June 21, the north pole is tilted
toward the Sun and the south pole is tilted away. Six months later the
situation is reversed. The tilt of the Earth's spin axis is exaggerated in
this figure

Left:
Click on the image to see an animated gif simulating the Earth's rotation on a
single day - June 21 - as seen from the Sun. The North Polar Cap is clearly
visible throughout this 24 hour period - the sun doesn't set. And Antarctica
cannot be seen - there, the sun doesn't rise. The images were generated by JPL's
Solar System Simulator; the animated gif is 349KB.

While the sun beats down on the North Pole from its highest point today, it
is pitch dark at the other end of the Earth. It is midnight at the South Pole
where the long night is just reaching its midpoint. The temperature there is a
frigid -57 deg. C and the sun won't rise again for 3 months.